The Global Positioning System (GPS) is a satellite-based navigation system made up of a network of 24 satellites placed into orbit by the U.S. Department of Defense. GPS was originally intended for military applications, but in the 1980s, the government made the system available for civilian use. GPS works in any weather conditions, anywhere in the world, 24 hours a day. There are no subscription fees or setup charges to use GPS.
GPS works using satellites that circle the earth twice a day in a very precise orbit and transmit signal information to earth. GPS receivers take this information and use triangulation to calculate the user's exact location. Essentially, the GPS receiver compares the time a signal was transmitted by a satellite with the time it was received. The time difference tells the GPS receiver how far away the satellite is. Now, with distance measurements from a few more satellites, the receiver can determine the user's position and display it on the unit's electronic map.
A GPS receiver must be locked on to the signal of at least three satellites to calculate a 2D position (latitude and longitude) and track movement. With four or more satellites in view, the receiver can determine the user's 3D position (latitude, longitude and altitude). Once the user's position has been determined, the GPS unit can calculate other information, such as speed, bearing, track, trip distance, distance to destination, sunrise and sunset time and more.
A GPS receiver can also be combined with navigation devices that provide mapping information. In these navigation devices, the location of the device, as determined by the GPS, is shown in relation to cartographic data, besides other type of data, on a display screen of the device. This cartographic data can be supplied to the navigation device through, for example, basemaps that are stored in the device. These basemaps contain data on major highways, thoroughfares, rivers, lakes, and borders, to name only a few of the possible cartographic features.
Navigation devices can also be used to calculate and provide a visual display of a course of travel between two or more waypoints. To accomplish this, a user can supply waypoint locations to the navigation device that calculates the course for the user. The navigation device can display the calculated course superimposed over a map that encompasses at least part of the calculated course.
Often times, however, the amount of information relating to the data contained in the basemaps used by the navigation device is not able to be fully displayed to the user. For example, a situation may arise where a calculated course is displayed having a relatively large map scale. In this situation, information that may be important to the user in considering whether to travel along the calculated course may not be made readily apparent. This type of information may include cartographic features such as the various grades (e.g., degree of slope) that would be encountered along the course, the type of terrain (e.g., rocky, sandy, hilly, unpaved) for the calculated course, and the presence of major obstacles (e.g., rivers, cliffs, gorges, swamps) along the calculated course. This type of situation may also arise when the calculated course is displayed having a relatively small map scale as well.